The present invention concerns a bath for the electro-deposition of gold and alloys particularly suited for the coating of pieces having irregular surfaces.
The invention also concerns the use of such baths for coating pieces intended to be used in the electronic industry, for instance transistor and diode headers and other printed circuit components.
It is well known in the plating art that the coating of irregularly shaped pieces is plagued with problems related to the variety of shapes of the surfaces to be plated. Thus, local plating current density variations are experienced which depend on the good or bad accessibility of the areas in contact with the electrolyte. Thus, for instance, the projecting angles of such pieces are subjected to current densities higher than average, whereas densities are below average around recessed angles. With classical plating baths, such a situation will result in thick deposits on the easily reached zones and thin deposits on the recessed areas. Or, in other words, when one desires to obtain a coating of a minimum thickness on the most recessed areas, one will have to accept the formation of an unnecessarily thick deposit on the exposed areas which condition, in the case of gold, is economically unfavorable.
Such a situation becomes even more critical when the pieces to be plated are too small to be hooked on a plating rack and the parts must be placed in a plating barrel. In such case, the pieces which tumble over each other contact the cathode only intermittently which results in further increasing the effect of relative variations in current density. In addition, it has been demonstrated that the pieces which are temporarily free from electric contact constitute dipoles in the electrolyte and that the metal of the anodic portion thereof tends to redissolve. This phenomenon thus further increases the thickness variations of the deposit.
It is already known that adding some mild reducing agents to aurocyanide electroplating baths, e.g., hydrazine or hydroxylamine and derivatives thereof substituted by organic groups (German DPS 1,215,467; 1,218,248; 1,222,347; UPS 3,770, 596; 3,783,111), tends to cancel the "dipole effect" mentioned above.
It is also known that some complexing agents such as, for instance, N-carboxymethyl-polyamines or N-phosphonylmethylpolyamines, namely ethylene diamine tetraacetic acid, EDTA (German DOS 2,220,621 [DEGUSSA], USP 3,783,111 [AURIC] and ethylene diamine tetramethylphosphonic acid, EDTP (UPS 3,770,596 [AURIC] can obviate the difficulties resulting from current density variations and improve the plating distribution. Said complexors also have the property of slowing down or preventing the co-deposition with the gold of certain metal impurities which may be present in the bath.
It is also known that the levelling power of a gold and gold-alloy plating bath is improved by adding, among other ingredients, an aldehyde and a polyamine substituted or unsubstituted, the aliphatic or aromatic substituents of which can comprise hydroxy groups, the effective quantity of such compound being comprised between 0.1 and 50 g/l of the plating solution (German DOS 2,244,434 [SCHERING]).
Finally, it is also known that the addition of water soluble arsenic compounds to gold plating baths improves the surface structure of the deposits and makes them more shiny and less porous (German DPS 2,042,127 [SEL-REX[, USP 3,423,295 [ENGELHARD]). It is, of course, desired to maintain the arsenic concentration as low as possible for various reasons such as safety. According to some Electronic Industry specifications, the concentration of arsenic in the bath will not be permitted to exceed a few mg/l.
It has now been found that none of the foregoing baths were satisfactory for the pure gold (practically 24 carat) barrel plating of some pieces used in said electronic industry. The distribution capacity of the baths comprising a reducing amine, a complexing agent and arsenic was not sufficient. In contrast, the platings obtained from baths containing an aldehyde, a polyamine in concentration above 0.1 g/l, a reduced amine and a small amount of arsenic has a tendency to discolor with heat. Such discoloration of surface decomposition constitutes a fundamental drawback for electronic applications since the plated pieces cannot be properly soldered. While this difficulty could be removed by increasing the arsenic content in the bath, the aresenic level in the deposits was too high to be acceptable.